Their heat-repelling tiles have been pecked by birds on the launch pad, dented by space dirt and scorched by the Earth's atmosphere.

But while the damage that accompanies each shuttle flight has long been a concern for NASA engineers, the insults suffered by the tiles had never produced the kind of catastrophic failure that led to the disintegration of Columbia on Saturday.

On Monday, NASA managers repeated their vow to find the problem that destroyed Columbia and seven astronauts and fix it. But a review of NASA's shuttle records over the years shows that it is easy to criticize the tile system -- fixing it will be a different matter.

Over the nearly 22 years of the operating shuttle program, the orbiters have averaged 144 separate impacts on the thermal tiles on each flight.

During that time, NASA has calculated, an average of 30 of those impacts produced cracks larger than 1 inch on every flight. On four missions before the Columbia disaster, shuttles lost whole tiles.

The orbiters have survived re-entry despite tile cracks, holes and scrapes largely because the damage was spread across several areas, or in parts of the craft that did not experience the highest temperatures during re-entry.

"We believe that we can lose a tile in different locations," said shuttle program manager Ron Dittemore. "We do not believe that will represent a loss of the vehicle."

But if that damage is concentrated in one sensitive part of the shuttle, NASA managers have long known what might result.

"We've lost tiles before and it did not imperil the mission," said U.S. Sen. Bill Nelson of Florida, who flew aboard Columbia in 1986. "But you can't lose a lot of them."

The tiles are of different sizes and made of different materials. The leading edges of the wings and a couple of other parts get the toughest coverings because they bear the highest temperatures. Those coatings were developed by LTV Aerospace and Defense, a predecessor of Vought Aircraft Industries.

But the whole shuttle can't be covered in the best tile. It's too heavy. The bulk of the replacement tiles for spots under and around the shuttle are made through the joint venture that manages the shuttle program. The tiles are produced by United Space Alliance, a joint venture of Boeing and Lockheed Martin, and are typically installed at Kennedy Space Center, where each shuttle is inspected and the damage cataloged after missions.

"The space shuttle program is a risky business and the tiles represent one of the riskiest parts of that business," said Alan Miller, a retired University of Washington professor of ceramic engineering, told the Seattle Post-Intelligencer. "They are the weakest link in the system."

Recognizing that, NASA and Lockheed Martin have spent years trying to tool a better tile.

NASA reports show that the agency has made several stabs at changing the materials in the tiles. To varying degrees, those efforts have made stronger, lighter tiles. But after more than 20 years of research, they are still far from perfect.

"They do a great job of insulating the shuttle, but they are somewhat fragile," Donald Kutyna, a retired Air Force four-star general who served on the presidential commission that investigated the 1986 loss of the shuttle Challenger, told the Los Angeles Times.

Beyond the fragility of the tiles, they must also be placed precisely on the orbiter, with just the right pressure and amount of adhesive. There were individually tailored standards for each of the more than 20,000 tiles on Columbia. A paper trail shows exactly who put each tile on, how much glue they used and how long they applied the correct amount of pressure. Those records are now being reviewed as part of the probe into Columbia's loss.

All those concerns about the tiles, from manufacture to installation, led NASA at one point to consider eliminating most of the tiles altogether. But ceramic blankets intended as replacements are either too heavy, or not tough enough to withstand re-entry temperatures above 2,300 degrees. Some parts of Columbia were covered in blankets, but technology hasn't invented a blanket that can handle all the stress the orbiter endures.

So they rely on the tiles.

"If you think about it, we've made a lot of improvements," said Cynthia Lodge, a NASA manager for program development at Kennedy who specialized in the tiles until 1993. "We've pretty much fixed all the tile and bonding issues."

That remains to be seen.

On Monday, Dittemore said he is hopeful that searchers will find a single, intact tile somewhere in Texas, but perhaps as far back on the shuttle's path as California.

With that tile, which is marked with a code showing exactly where it fit on the orbiter, NASA would be able to determine whether it was in an area that was hit by foam from the external fuel tank.

If it came from a different area, investigators could review the records from Columbia's last refurbishment in 1999 to determine who put that tile on and whether it was done properly.